Various attempts have been disclosed in the art to improve the quality of vegetables which have been stored frozen by way of the freezing process applied.
U.S. Pat. No. 3,736,154 describes a process of ultraslow freezing which discloses the maintenance of intact cell membranes in the product by way of a freezing regime with a cooling rate of about 0.1 to 0.3° C. per hour. This process is disclosed as achieving dehydration of the inner cell as water within the cell moves outside the cell membrane where it freezes without the destruction of the cell membrane.
Unfortunately this process is not a viable approach to the commercial preparation of frozen vegetables. Not only does this process not tolerate a blanching step which is necessary as a microbiological and enzyme deactivation step in modern vegetable processing, but also this process takes several days to complete.
U.S. Pat. No. 6,096,361 discloses a similar method of preservation wherein food is relatively rapidly cooled from room temperature to close to the freezing point and then slowly cooled at a gradual cooling rate of 0.01 to 0.5° C./hour to below the freezing point. This non-frozen preservation method may be then followed by a rapid freezing treatment to achieve a food wherein the outer cells of the food are frozen and the inner cells preserved in a non-frozen state. It is disclosed that free water moves from the intracellular fluid to the extra cellular fluid, resulting in the simultaneous dilution of the extra cellular fluid and concentration of the intracellular fluid, which makes it easier for the extra cellular fluid to freeze and, conversely, more difficult for the intracellular fluid to freeze.
An alternative freezing process of the prior art for improving frozen vegetable quality is described in European Patent 0 554 468 B1 in which potatoes are cooked and frozen. Freezing is described as a 2 step process wherein, in an initial step, the core of the potatoes is kept at the crystallisation stage of water for a period of 15 to 60 minutes. In a second step deep-freezing is continued until a storage temperature of −20° C.
The present invention also addresses the technical problem of providing high quality frozen vegetables to the consumer, and more particularly to provide frozen vegetables which, when thawed, give rise to a product texture and appearance which closely resembles that of fresh vegetables.
The applicants believe that a significant cause of texture loss in frozen vegetables is as a result of tissue damage resulting from extracellular ice formation, therefore the problem of the invention is more particularly identified as providing a reduction in extracellular ice formation during freezing.
The solution to this problem provided by the present invention resides in a novel freezing process which can practically eliminate ice formation outside the cell wall of the vegetable tissue and so provide a texture, appearance and thus product quality that was not previously possible in frozen vegetables.